(1) Field of the Invention
The present invention concerns a method and equipment for heat recovery from exhaust gas from a process plant, for instance raw-gas from an electrolysis plant for the production of aluminium. Such exhaust gas may, before it is cleaned, contain dust and/or particles that will form deposits on the heat recovery equipment and thus reduce the efficiency of the heat recovery to an undesired, low level.
(2) Description of Related Art
Various industrial processes produce exhaust gases that can be contaminated by particles, dust and other species that can cause fouling in energy recovery equipment. Such fouling will result in reduced efficiency, and will require extensive maintenance such as cleaning of the surfaces exposed to the gas flow. Thus, energy recovery units are placed downstream a gas cleaning plant, after the gas has been cleaned. With respect to optimising the energy recovery, it is of interest to arrange the recovery units as close to the industrial process as possible, where the energy content in the exhaust gas is at its maximum. This implies that the energy recovery units have to be arranged upstream the gas cleaning plant, because such plants are often localized relatively distant to the industrial process.
U.S. Pat. No. 4,339,249 discloses a heat exchanger for recovery of heat energy from dust containing waste gases. The exchanger is constructed to recover much of the dust entrained in the gases and includes a hollow duct through which the waste gases pass, and which contains first and second tube bundles arranged one after the other and a dust collection surface between them. The heat content in the waste gases is transferred to water passing through the two tube bundles and dust is deposited on the dust collection surface. The tubes in the bundles are arranged in a serpentine configuration, and the first bundle is constructed of a smooth surface tube arranged to remove heat from the dust containing gases upstream the dust collection surface. Thus, when the gases reach the finned tube bundle, it is stated in the publication that no deposition (and clogging) of the narrow spaces between the fins will occur. Thus from this citation it is learned that the dust containing gases should be treated in a first cooling step and then in a separating step before it is entered into the section comprising a second finned tube bundle of a flanged tube (or tubes).
For example, exhaust gas from aluminium electrolysis furnaces contains large amounts of energy at a relatively low temperature level. This energy is currently utilized only to a small extent, but it can be used for heating purposes, process purposes and power production if technically and economically acceptable solutions for heat recovery are established. The temperature level achieved in the heated fluid is decisive to the value and usefulness of the recovered thermal energy. The heat should therefore be extracted from the exhaust gas at as high an exhaust gas temperature as possible. Other examples of industrial processes that produce large exhaust gas volumes containing dust/particles are: Ferro-, alloy- and other smelting plant industries that typically operates with dust-containing exhaust gases at 300° C. and higher, and the low temperature section in waste incineration (i.e. economizer and air preheating sections) that typically operate at 300° C. and lower.
The exhaust gas from electrolysis furnaces is transported through a suction system by means of fans, and the power consumption of the fans depends on the volume flow of exhaust gas and the pressure drop in the system. The power consumption can be reduced by a reduction of these quantities. Cooling the exhaust gas will contribute to reduced volume flow rate and pressure drop, with reduced fan power as a consequence. The largest reduction in pressure drop is achieved by cooling the exhaust gas as close to the aluminium cells as possible.
When improving or scaling-up an industrial process, for instance increased current (amperage) in relation to a given cell-design in a aluminium electrolysis plant, the raw-gas temperature and thus its pressure inside the superstructure will increase as there will be more heat present above the top of the cell. This can result in cell puncture, i.e. the same level of pressure will be present at the inside as that of the outside of the cell. By such puncturing, the emission of process gases to the production hall will increase.
This problem can be solved in three ways:                Enchange the encapsulation at the cell top, which can be difficult in practice.        Increase the suction by installing higher fan capacity. To avoid large pressure drop in the raw gas channels, these must be increased in size as well. The gas cleaning plant will have to be re-designed to avoid reduced efficiency or overloading components in the gas cleaning process. In total this solution will be expensive with regard to both investment and operation costs.        Cooling of the raw gas upstream the fans together with heat recovery, a solution that will reduce both the raw gas volume flow rate and the pressure drop in channel system and gas cleaning plant. The suction can thereby be increased without the need of changing the dimensions of channels and gas cleaning plant.        